In a 4-cycle internal combustion engine, in general, as shown in FIG. 4, a valve guide 120 made of cast iron for supporting a stem 111 of intake and exhaust valves 110 and a valve seat 130 made of a hard material such as a sintered alloy are fitted in predetermined positions of a cylinder head 100. As shown in FIG. 5, the valve seat 130 usually has a seat surface comprising a mouth surface 131 which has a vertical angle of 120.degree. in an imaginary cone described around an axis 112 of the stem 111 as an axis of rotation, a valve contact surface 132 which has a vertical angle of 90.degree. in the imaginary cone and an inner surface 133 which has a vertical angle of 60.degree. in the cone, provided there are some valve seats wherein the seat surface comprises only the valve contact surface 132 having a vertical angle of 90.degree.. Where it is necessary to maintain air-tightness against a violent motion and a high compressive force such as in the intake and exhaust valves in the cylinder head of an internal combustion engine, close attention is paid to finishing the inner surface of the guide bore of such valve guide and the seat surface of such valve seat, and particularly a high roundness of the valve seat surface itself and an alignment of high accuracy relative to the valve guide bore are required in finishing the valve seat surface.
In view of the foregoing requirements, various machining methods have heretofore been adopted, which are broadly classified into the following three methods.
(1) Using a jig borer or the like, the valve guide bore is finished with a reamer, then the reamer is removed and a measuring tool such as a dial gauge is attached, followed by measurement while feeding the spindle to effect alignment between the valve guide and the valve seat, and then the measuring tool is replaced by a chamfering cutter and the valve seat surface is chamfered by this cutter.
This method is unsuitable for a multiple spindle machining, and even in machining with a single spindle a fairly large amount of time is required, so the productivity is low and labor saving is not attainable. Therefore, this method has been utilized mainly in small volume production, and is not suitable for medium and large volume production.
(2) The valve guide bore is reamed in a previous step using a separate machine tool, then a pilot bar integral with a chamfering cutter is inserted in the reamed guide bore and the valve contact surface of a valve seat is manually chamfered.
According to this method, at every chamfering of the valve seat surface, it is necessary to effect alignment with the guide bore. Consequently, in case the valve seat is made of a relatively soft material, the working efficiency is not so much affected, but in the case of a hard material, more time is required and it is necessary to machine beforehand the mouth surface and inner surface of the valve seat. Besides, because of manual work, force may be applied non-uniformly in the chamfering operation, so that the alignment of the seat surface relative to the guide bore and the roundness of the seat surface itself are not always perfect. Consequently, after-steps including valve running operation have been required in many cases.
(3) As shown in FIG. 6, a cylinder head 200 is set, with a jig, in a predetermined position of a machine tool, then an outer spindle 270 supporting a cutter head 250 on the outer periphery of which is fixed a chamfering cutter 240, and an inner spindle 280 to which is secured a reamer 260, are moved forwardly integrally with each other to approach the cylinder head 200, and when they have reached predetermined positions, only the inner spindle 280 is moved forwardly while being supported by the inner surface of the cutter head 250 and the guide bore of a valve guide 220 is finished with the reamer 260, and then the outer spindle 270 is moved slightly forward to chamfer the seat surface of the valve seat 230.
This method is suitable for medium and large volume production and has been mainly adopted at present. However, in the foregoing methods (1) and (2), even when there is a small deviation in the fitting condition of the valve guide relative to the cylinder head, that is, even when the guide bore is not positioned as designed, the chamfering of the valve seat is performed after making a setting corresponding to the deviation of the guide bore by alignment with the guide bore, while in the method (3), the guide bore and the seat surface are machined according to predetermined dimensions from reference positions regardless of variation in the guide bore position. As a result, when machining a prepared bore for the valve guide, if the prepared bore is not positioned accurately, there will occur non-uniformity in the radial allowance, namely, the so-called thickness deviation, at the time of reaming and this thickness deviation acts on the cutting edge of the reamer as a non-uniform, cutting reaction force in the radial direction, so that as the long and slender reamer is moved forwardly, not only the inclination of the reamer increases but also the degree of eccentricity with respect to the prepared bore increases. Therefore, the concentricity between the guide bore and the valve seat has heretofore not been easily attainable. Besides, the corresponding machine tool is inevitably a double coaxial spindle type because of the necessity of feeding the reamer and the cutter separately, so that the diameter of the outer spindles becomes larger and hence the number of spindle per machine becomes relatively small. Therefore, in order to finish a workpiece such as particularly a multiple-cylinder engine cylinder head with high working efficiency, a plurality of work stations have heretofore been needed.